Vapor deposition method and apparatus



Feb. 14, 1961 c. A. BAER ETAL 2,971,862

VAPOR DEPOSITION METHOD AND APPARATUS Filed April 26, 1957 INVENTORS Ckor/er A, Bath Eda/0rd SCaa J l [mm-w. A ATToRNp United States PatentVAPOR DEPOSITION METHOD AND APPARATUS Charles A. Baer, Needham, WallaceF. Bugbee, Boston, Edward S. Candidus, Cambridge, and Philip J. Clough,Reading, Mass., assignors to National Research Corporation, Cambridge,Mass, a corporation of Massachusetts Filed Apr. 26, 1957, Ser. No.655,384 '14 Claims. (Cl. 117-61) This invention relates to coating andmore particularly to the coating of heat-sensitive substrates, such aspaper with aluminum, to provide a shiny surface having both functionaland decorative utility.

Prior workers in the art of vacuum coating have been striving for manyyears to coat paper and similar materials with aluminum under conditionswhich are economically competitive with aluminum foil-paper laminates.To date none of these Workers have been successful.

Accordingly, it is a principal object of the present invention toprovide an improved process and apparatus for metallizing heat-sensitivesubstrates such as paper to produce a shiny aluminum coating on thesubstrate.

Another object of the invention is to provide an improved process andapparatus which produces such a coating under conditions which make iteconomically competitive with aluminum foil-paper laminates.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation and the order of one or more of such steps withrespect to each of the others and the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed disclosure, and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing which is a schematic,diagrammatic, sectional view of one preferred embodiment of theinvention.

The present invention will be principally described in connection withits utility in the vacuum aluminizing of paper to produce a materialwhich is economically competitive with aluminum foil-paper laminates.However, it will be apparent that the utility of the present inventionextends far beyond the specific application described.

In the aluminizing of paper it is essential that the paper have asmooth, continuous surface to receive the aluminum so that a highlyreflective surface film will result. This is due to the fact that thevacuum-deposited aluminum film is extremely thin (on the order of only afew millionths of an inch) and accordingly it will follow exactly thesurface of the paper.

An additional important fact is that paper always contains a substantialpercentage of water. Dehydrated paper is extremely brittle andessentially useless in most commercial applications. Accordingly, watervapor will come out of the paper when the paper is exposed to a vacuumon the order of a millionth of an atmosphere, which is the degree ofvacuum preferred for vacuum aluminizing. Large amounts of water vaporare extremely detrimental to the vacuum coating operation. Attempts topartially dry the paper before aluminizing have not been uniformlysuccessful and in most cases have necessitated subsequent treatment ofthe paper to put additional water back into it.

We have discovered that the difiiculties enumerated 7 above can belargely avoided without resorting to any significant drying of the paperor to other complex pre-- treatment. This is partly achieved in thepresent invention by coating the surface to be aluminized with asubstantially continuous film which can serve as at least a temporarymechanical barrier to water vapor and gases attempting to diffuseoutwardly from the paper through the coating. This coating is preferablyformed of a filmforming material such as polyvinyl acetate, latex or thelike which can be applied as a water suspension and dried to form asmooth, continuous film. The process additionally includes the step ofpassing the thus-prepared paper through an intermediate vacuum chamberwherein both surfaces thereof are exposed for only a.

brief period of time to a vacuum on the order of 10 to 200 microns Hgabs. or at least in excess of 10 microns Hg abs. This exposure to theintermediate vacuum is primarily for the purpose of removing largequantities of physically entrapped air held between the layers of. paperin a paper roll and within the voids necessarily of the invention thesubstrate passes through the high vacuum chamber in less than about 1second and pref-: erably in less than of a second or of a second. Duringthis extremely brief interval of time, it is sub-- jected to a highintensity stream of aluminum vapors' which provides on the exposedsurface of the paper an aluminum film having a total reflectance of 70percent.

01' more.

The time during which the substrate is in the coating,

chamber is so short that practically none of the sorbed water isliberated from the paper. Any small portion of water which does manageto become desorbed is believed to be reacted almost immediately with thehigh concentration of aluminum vapors adjacent the paper surface.Accordingly, the paper loses substantially none of itswater content andreceives a shiny, adherent film of aluminum on one surface.

In order to permit the short coating time, the substrate is preferablypassed through the coating chamber at a high rate of speed, on the orderof one to two thousand feet per minute, and the distance through whichthe substrate passes while in the coating chamber is preferably keptshort (e.g. on the order of a foot or less). The source of aluminumvapors is preferably maintained at an elevated temperature (above 1200C., for ex-- ample) and the area of molten aluminum in position tocoatthe substrate is preferably on the order of at least 10 percent of thearea of paper being coated.

In a preferred form of apparatus embodying the invention, the substrateis fed into and through the coating chamber while being supported by andheld in contact with a continuous, impermeable, moving surface, thesurface preferably comprising a large cooled drum which can serve as oneportion of the wall defining the coating chamber. Accordingly, the drumsurface serves to seal the back surface of the paper during at leastmost of the time the paper is in the coating chamber and only thesurface to be the coating chamber. As mentioned previously, this surfaceto be coated is preferably exposed to the high vacuum for a very smallfraction of a second.

After coating, the substantial dehydration of the paper is prevented byrerolling the paper in the intermediate chamber within a second-or twoof the time that it is Patented Feb. 14, 1961 In a preferred embodimentcoated is exposed to the high vacuum in' intermediate chamber to theatmosphere Within a few seconds of coating. Throughout this invention,wherever the amount of volatiles liberated .is minimized, as a necessarycorollary, the pumping requirements for. maintaining, the desired vacuumare minimized.

Referring now to the drawings, there is illustrated a diagrammatic,schematic, sectional representation of one preferred embodiment of theinvention- As illustrated, the apparatus includes an intermediate vacuumchamber 10 arranged to be evacuated by a high capacity vacuum pumpingsystem 12.. This chamber 10 has a vacuumtight cover 14 which can belifted up to permit loading,

andunloading rolls of paper 16. The apparatus also includes a vacuumcoating chamber 18, including a,

source of aluminum vapors 20, this source preferably comprising aninduction heated carbon crucible of the type described andclaimed in thepatent of Clough et al. 2,665,223. A suitable power. supply is indicatedat 22 with power leads 24. Between the intermediate vacuum chamber 10and the high vacuum coating chamber 18, there is provided in a preferredembodimenta large drum 26, cooled by cooling coil 27, which almost com-'pletely fills an opening 28 between these two chambers. This opening isdefined in part by curved surfaces 30 which cooperate with the drumsurface to form high impedance paths substantially preventing transferof large quantities of gas from the intermediate vacuum chamber 10 t thehigh vacuum coating chamber 18. This drum is so positioned, with respectto a pair of guide rolls 32, that the paper web 16 is pressed intocontact with the drum 26 and held in contact with this drum while it iscarried into the coating chamber 18, through the stream of aluminumvapors (indicated at 2011) and back out of the coating chamber. Theimpedance paths created by the curved surfaces or shrouds and the drumthus constitute apertures through which the paper to be coated mustpass. For highest impedance the aperture must be minimized; however,there must be sufficient clearance for the paper to pass, while incontact with the drum, without touching the shrouds. Moreover, when itis necessary to join one length of paper with another by means of a lapjoint, the aperture must be large enough to allow two thicknesses ofpaper to pass without interference. Accordingly, it is preferred thatthe spacing between the shrouds and the drum be made adjustable topermit optimum spacing for different thicknesses of paper.

In a preferred use of the invention, the paper to be aluminized is firsttreated by having applied thereto a continuous filmto give the paper asmooth, shiny surface to receive the aluminum coating. The paper, afterreceiving this continuous film, is preferably rolled up so that thisfilm is always on the outside of the roll; thus, as roll 16 is beingunwound in the intermediate vacuum chamber 10, its outer surface isessentially continuously sealed by the continuous film. If desired, theends of the roll can also besealed by closely fitting metallic endplates, for example. Thus the paper, prior to aluminizing, is exposed tothe vacuum only during the rather brief time when it is traveling fromthe roll onto. the drum 26., Similarly after metallizing, the paper is'rewound again with the metallized surface facing away from the axis ofthe rewound roll.

In a preferred embodiment of the invention, vacuum pumps 12 and 19 arepreferably high capacity mechanical vacuum pumps or steam ejectors. Pump12 preferably can maintain the intermediate vacuum chamber at a pressureon the order of to 200 microns, this vacuum being primarily for thepurpose of removing air which is entrapped between the layers of paper16.0r within the voids necessarily present within the paper. The highvacuum coating chamber 18 is maintained by pumping at a pressure lessthanabout 20 microns, preferably less than,l0 microns. A pressure,of;rnuch lessthan 10 microns is not necessary if the source of aluminumvapors is quite close to the paper being coated (e.g. about 6 inches)and the area of aluminum vapor source is relatively large with respectto the area being coated. For example, if the source is within about 6inches of the paper and the area of the hot aluminum in the source isabout 20 to 50 percent of the area of paper being coated at any instantof time, the pressure can be as high as 15 to 20 microns while stillobtaining a shiny, adherent coating.

In a preferred embodiment of the invention, the high vacuum coatingchamber 18 includes a removable bottom 36 which carries the electricalconnections and supports the crucibles 20. Thus the supply of cruciblescan be rapidly changed by removing the whole assembly 36-20 andreplacing this assembly with a spare, containing crucibles loaded withaluminum. A shield 38 is also preferably provided to permit thecrucibles to be brought up to operating temperature without exposing thesubstrate to radiation from the crucible until the, aluminum is at thedesired evaporating high tempertaure on the order of l200 C. to 1300 C.

In operating the device described above, the substrate is preferablymoved through the high vacuum coating.

chamber at a relatively high speed on the order of 1000 to 2000 feet perminute. When the lineal dimension of the exposed area of the substrate(i.e. the areajbetween.

th ends of the two shrouds 30) is on the order of one linear foot, it isapparent that the time in which any unit area of the substrate remainsin the high vacuum cham ber is extremely short, 0n the order of to of a,

second depending upon the speed of the substrate. Where the distancebetween the shrouds is minimized, thus rnini..-' mizing the exposed areaof the substrate, it is apparent. that more of the exposed area of thesubstrate is in the,

ing have a reasonably shiny surface and that it have suf-v ficientresistance to the passage of water vapor and .gases to preventsubstantial release of water vapor and other sorbed volatiles in thesmall fraction of a second that the paper is exposed to the high vacuumof the coating chamber. Additionally the coating material shouldpossessta) good adhesive properties (i.e. be capable of.

bonding firmly to the substrate), (11) be capable of drying to a hardfilm within a reasonable length of-time, and (c) not outgas appreciablywhen subjected to low pressures necessary for the vacuum deposition ofalumi-.

num. There are numerous coating materials which meet the aboverequirements. For instance, thermosetting'.

resins such as the phenol-formaldehydes, urea or melamine formaldehydes,alkyd resins, silicone resins, and

epoxy resins can be employed. These resins may be. used along or theymay be modified. Likewise certain,

organic lacquers and varnishes are suitable. Water glass can also beutilized.

Equally numerous other mechanical embodiments can be employed. Forexample, the drum can be replaced by a moving'belt although the drum ismuch preferred from the standpoint of mechanical simplicity.

While the invention has been specifically described in connectionwithpaper, it can also be utilized to advantage in the coating ofheat-sensitive plastics such as cellophane, cellulose acetate and thelike which contain ap-f.

preciable quantities of sorbed water, plasticizer, or other volatiles.Since the high'speed coating of the present invention permitsthesubstrate to bemoved through the, high vacuum coating chamber in such ashortperiod,

of time, no substantial outgassing of the substrate can take place.Accordingly, the present invention can be utilized to coat materialswhich, in the past, have universally been considered as requiringextensive outgassing prior to coating.

The physical volume of the apparatus is preferably .kept to an absoluteminimum consistent with the space requirements for the bulk of thematerial to be placed within its vacuum chamber. Thus, when theapparatus is to be utilized for semi-continuous coating as illustrated,the chamber 16 is preferably made only slightly bigger than necessary tohold one completely full roll. Accordingly, as one roll is unwound andthe othervis rewound, the roll of increasing diameter will graduallytake up the space previously occupied by the roll of decreasingdiameter. Equally, the substrate may be passed from atmosphere into thecoating chamber through a series of locks of the type shown in thecopending Bugbee application, Serial No. 565,599, filed February 15,1956. With this arrangement the coated substrate isalso preferablycontinuously taken out of the vacuum chamber and rewound outside of anyevacuated portion of the apparatus.

4 Since certain changes may be made in the above process and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description, or shown inthe accompanying drawing, shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. A process for coating aluminum onto a heat-sensitive substrate, suchas paper, which contains adsorbed and absorbed volatiles, whichcomprises the steps of exposing the substrate to a vacuum of betweenabout 20 and about 290 microns Hg abs. for a period sufiicient to removesubstantial quantities of entrapped gases, subsequently passing thesubstrate into and out of a coating chamber maintained at a pressurebelow about 20 microns Hg abs., exposing the front surface of thesubstrate to aluminum vapors While it is in the coating chamber, theconcentration of aluminum vapors being sufficient to produce a coatinghaving at least 70 percent reflectance, the substrate moving at such arate that no unit area of the substrate is exposed to the high vacuum ofthe coating chamber for a period in excess of second, back surface ofthe substrate being held in contact with an impermeable surface movingat the.

same speed as the substrate for substantially all of the time that thesubstrate is in the high vacuum coating chamber, whereby the content ofsorbed volatiles in the substrate remains substantially unchanged.

2. A process for coating aluminum onto paper which contains adsorbed andabsorbed Water and entrapped air, the process comprising the steps ofexposing the paper to a vacuum of between about 20 and about 200 micronsHg abs. for a period sufiicient to remove substantially all of theentrapped air without removing substantial quantities of sorbed water,subsequently passing the paper through a coating chamber maintained at apressure below about 20 microns Hg abs., exposing the front surface ofthe paper to a source of aluminum vapors in the coating chamber,maintaining the source of aluminum vapors at a temperature above 1200C., the source having sufiicient area to evaporate at least 0.5 gram ofaluminum per minute for each inch of paper width passing through thecoating chamber, the paper being moved through the coating chamber atsuch a rate that no fiber of the paper is exposed to the vacuum of thecoating chamber for a period in excess of second, the back surface ofthe paper being held in contact with an impermeable surface moving atthe same speed as the paper for substantially all of the time that thepaper is in the high vacuum coating chamber.

3. A process for coating aluminum onto a heat-sensitive substrate, suchas paper which contains adsorbed and absorbed volatiles and entrappedair, the process comprising the steps of exposing the substrate to avacuum of between about 20 and about 200 microns' Hg abs. for a periodsufficient to remove substantial quantities of entrapped air withoutremoving substantial quantities of sorbed volatiles, subsequentlypassing the substrate through a coating chamber maintained at a pressurebelow about 20 microns Hg abs., exposing the front surface of thesubstrate to a source of aluminum vapors in the coating chamber,maintaining the source of aluminum vapors at a temperature above 1200C., the source having suflicient area to evaporate at least 0.5 gram ofaluminum per minute for each inch of. substrate width passing throughthe coating chamber, back surface of the substrate being held in contactwith an impermeable surface moving at the same speed as the substratefor substantially all of the time that the substrate is in the highvacuum coating chamber, the substrate being moved through the coatingchamber at such a rate that no fiber of the substrate is exposed to thevacuum of the coating chamber for a period in excess of second.

4. A process for coating aluminum onto a heat-sensitive substrate, suchas paper, which contains adsorbed and absorbed volatiles which comprisesthe steps of exposing the substrate to a vacuum above 20 and below about200 microns Hg abs. for a period sufficient to remove substantialquantities of entrapped air, subsequently passing the substrate into andout of a coating chamber maintained at a pressure below about 20microns, exposing the substrate to aluminum vapors while it is in thecoating chamber, the concentration of aluminum vapors being sufficientto produce a coating having at least 70 percent reflectance, thesubstrate moving at such a rate that no fiber of the substrate isexposed to the high vacuum of the coating chamber for a period in excessof second, and maintaining the back surface of the substrate in contactwith an impermeable surface moving at the same speed as the substratefor the whole time that the substrate is in the high vacuum coatingchamber, whereby the content of absorbed volatiles in the substrateremains substantially unchanged.

5. A process for coating aluminum onto a heat-sensitive substrate, suchas paper, which contains adsorbed and absorbed volatiles which comprisesthe steps of exposing the substrate to a vacuum of no lower than 20microns Hg abs. for a period sufiicient to remove substantial quantitiesof entrapped air, subsequently passing the substrate into and out of acoating chamber maintained at a pressure below about 20 microns,exposing the substrate to aluminum vapors while it is in the coatingchamber, the concentration of aluminum vapors being sutficient toproduce a coating having at least 70 percent reflectance, the substratemoving at such a rate that no fiber of the substrate is exposed to thehigh vacuum of the coating chamber for a period in excess of second, andmaintaining the back surface of the substrate in contact with animpermeable surface moving at the same speed as the substrate for thestrate is in the high vacuum coating chamber, and preventing prolongedexposure of the mass of substrate to any substantial dehydratingconditions by rerolling the substrate within one second of the time ofapplication of the aluminum coating thereto, whereby the content ofabsorbed volatiles in the substrate remains substantially unchanged.

6. A process for coating aluminum onto a surface of a heat-sensitivesubstrate, such as paper, which contains adsorbed and absorbedvolatiles, the surface having been first provided with a continuous filmwhich creates a substantial barrier to the passage of vapor and gases,the process comprising the steps of exposing the substrate to a vacuumsufiicient to remove substantial quantities of entrapped gases,subsequently passing the substrate into and out of an alumim'zingchamber maintained whole time that the sub-.

at a pressure-below, about 20 microns Hg,abs. ,.exposing the film-coatedside of the substrate to aluminum vapors while it is in the aluminiz ingchamber, the concentration of aluminum vapors being suficient to producean aluminum coating having at least 70 percent reflectance, thesubstrate moving at such a rate that no unit area of the substrate isexposed to the high vacuum of the aluminizing chamber for a period inexcess of /s second, back surface of the substrate being held in contactwith an impermeable surface moving at the same speed as the substratefor substantially all of the time that the substrate is in the highvacuum coating chamber, whereby the .content of sorbed volatiles 1n thesubstrate remains substantially unchanged.

7. A process for coating aluminum onto a heat-sensitive substrate, suchas paper, which contains adsorbed and absorbed volatiles, whichcomprises the steps of exposing the substrate to a vacuum of betweenabout 20 and about 200 microns Hg abs. for a period suflicient to removesubstantial quantities of entrapped gases, subsequently passing thesubstrate into and out of a coating chamber maintained at a pressurebelow about 20 microns Hg abs., exposing the substrate to aluminumvapors while it is in the coating chamber, while maintaining the sideofthe substrate which is not to be coated in intimate contact with animpermeable mechanized moving surface for substantially all of theperiodduring which the substrate is within the coating chamber, the

concentration of aluminum vapors being sufficient to produce a coatinghaving at least 70 percent reflectance, the substrate moving at such arate that no fiber of the substrate is exposed to the high vacuum of thecoating chamber for a period in excess of 1 second, whereby the contentof sorbed volatiles in the substrate remains sub-' stantially unchanged.

8. A process for coating aluminum onto a surface of a heat-sensitivesubstrate, such as paper, which contains adsorbed and absorbedvolatiles, the surface having been first provided with a continuous filmwhich creates a substantial barrier to the passage of vapor and gases,the process comprising the steps of exposing the substrate to a vacuumof above 20 microns Hg abs. for a period sufficient to removesubstantial quantities of entrapped gases, subsequently passing thesubstrate into and out of an aluminizing chamber maintained at apressure below about 20 microns Hg abs., exposing the film-coated sideofthe substrate to aluminum vapors while it is in the aluminizingchamber, the concentration, of aluminum vapors being suflicient toproduce an aluminum coating having at least 70 percent reflectance, theside of the substrate which does not have the film provided thereonbeing maintained in intimate contact with an impermeable moving surfacefor substantially all of the period during which the substrate is Withinthe aluminizing chamber whereby the content of sorbed volatiles in thesubstrate remains substantially unchanged.

9. A process for coating aluminum onto a surface of a heat-sensitivefront surface of the substrate, such as paper, which contains adsorbedand absorbed volatiles, the surface having been first provided with acontinuous film which creates a substantial barrier to the passage ofgases, the process comprising the steps of unrolling the substrate froma roll while exposed to a vacuum of between about 20 andabout 200microns Hg abs., the substrate having been rolled so that the surface ofthe substrate upon which the film has been provided faces away from theaxis of the roll, passing the substrate immediately into a coatingchamber maintained at a pressure below about 20 microns Hg abs.,exposing the substrate to aluminum vapors While it is in the coatingchamber, the concentration of aluminum vapors being sufficient toproduce an aluminum coating having at least 70 percent. reflectance,back surface of the substrate being held in contact withan impermeablesurface movatthe samespeed as the substrate for substantially all of thetime that the substrate is in the high vacuum I coating chamber, thesubstrate moving at such a rate that no unit area of the substrate isexposed to the high vacuum of the coating chamber for a period in excessof /5 second, removing the substrate from the coating chamber intovacuum of between about 20 and about 200 microns Hg abs., andimmediately winding the substrate upon a roll in the vacuum in such amanner that the coated surface of the substrate upon the roll faces awayfrom the axis of the roll.

10. Apparatus for coating a substrate by vacuum vapor depositiontechniques which comprises an intermediate vacuum chamber and a highvacuum coating chamber, said chambers being connected by means of atleast one seal through which the substrate can be passed from onechamber to'the other chamber, said seal being defined by a cylindricalgas impermeable rotatable support coaxially positioned within twoarcuately curved walls,

with said cylinder as it passes from said intermediate vacuum chamberthrough said seal to said high vacuum coating chamber and subsequentlyback to said intermediate vacuum chamber, said holding means serving tomaintain said substrate under tension so as to maintain one side of saidsubstrate in contact with said rotatable support while it is in the highvacuum chamber, means for rotating said support at a surface velocityequal to linear velocity of said substrate, and means for providing asource of coating vapors within said high vacuum chamber.

11. Apparatus for coating a substrate by vacuum vapor depositiontechniques, said apparatus comprising an intermediate vacuum chamber, ahigh vacuum coating chamber, a continuous surface extending from withinsaid intermediate vacuum chamber into said high vacuum chamber, meansfor pressing said substrate into contact with said surface and means formoving said surface into and out of said high vacuum chamber while saidsubstrate is held in contact with, and moves with said surface, saidmoving means being arranged to move said substrate at a speed in excessof 1000 feet per minute, said pressing means being arranged to hold saidsubstrate in contact with said moving surface during substantially thewhole time that the substrate is moved through the high vacuum chamber,vacuum pumping means for maintaining said intermediate vacuum chamber ata pressure between about 20 and about 200 microns Hg abs., vacuumpumping means for maintaining said coating chamber at pressure less thanabout 20 microns Hg abs., and means for vaporizing aluminum in saidcoating chamber.

12. An apparatus for coating one side of a paper substrate by metalvapor deposition, which paper is received fromand returned to a lowvacuum chamber, said apparatus comprising a high vacuum chamberadjoining the low vacuum chamber from which the paper is fed from andreturned to, said high vacuum chamber containing a means for generatingthe metal vapors to be deposited on the paper, a large rotatablecylinder defining a portion of the side of the high vacuum chamber nextadjoining the low vacuum chamber, the remaining portion of said sidecontaining stationary shrouds which tightly enclose said cylinder suchthat only space is left between the curved sides of said cylinder andthe stationary shrouds to permit the passage of tWo thicknesses of thepaper being coated while in contact with the cylinder, without contactwith the stationary shrouds, means in the low vacuum chamber for guidingthe paper substrate on to the cylinder as the paper is fed to the highvacuum chamber and off the cylinder as the paper is returned from thehigh vacuum coating chamber, said guiding means paper surface exposed tothe high vacuum is being contacted by the direct flow of metal vaporthan is not being so contacted.

such that more in the chamber the 13. An apparatus for coating one sideof a moving comprising a high vacuum chamber maintained at a pressure nogreater than 20 microns Hg abs.,

a source of coating vapors within the high vacuum chamber and a meansprovided for introducing the substrate, supporting and cooling thesubstrate while being coated and removing the substrate subsequent tocoating, said means comprising a, large rotatable supported water-cooledcylinder positioned in one of the walls of the high vacuum chamber nextadjoining the chambers from which the paper is introduced and returned,the cylinder being surrounded by portions of the wall so as to leaveonly two small apertures between the cylinder and the walls, one on eachside of the cylinder, through which the paper is fed to the high vacuumchamber and removed from the high vacuum chamber, the paper being incontact with the cylinder throughout its presence in the high vacuumchamber, the cylinder being rotated at a circumferential speed equal tothat of the moving paper, the source of coating vapors being positionedso as to produce vapors which impinge directly upon the paper as itmoves upon the cylinder, the water-cooled cylinder serving to cool thesubstrate as the vapors impinge thereupon, thus permitting the formationof a film of coating upon the paper substrate.

14. The apparatus of claim 10 wherein the source of aluminum vaporscomprises a means for supporting a body of high temperature liquidaluminum having a surface exposed to the paper which is at least 10percent of the area of the paper in position to receive aluminum vaporsat a given instant of time.

References Cited in the file of this patent UNITED STATES PATENTS2,402,269 Alexander et al. June 18, 1946 2,562,182 Godley July 31, 19512,665,224 Clough et al. Jan. 5. 1954 2,665,228 Stauffer Jan. 5, 19542,687,361 Traub Aug. 24, 1954 2,702,760 Barth Feb. 22, 1955 2,795,522Johns June 11, 1957

1. A PROCESSE FOR COATING ALUMINIUM ONTO A HEAT-SENSITIVE SUBSTRATE,SUCH AS PAPER, WHICH CONTAINS ADSORBED AND ADSORBED VOLATILES, WHICHCOMPRISES THE STEPS OF EXPOSING THE SUBSTRATE TO A VACUUM OF BETWEENABOUT 20 AND ABOUT 200 MICRONS HG ABS. FOR A PERIOD SUFFICIENT TO REMOVESUBSTANTIAL QUANTITIES OF ENTRAPPED GASES, SUBSEQUENTLY PASSING THESUBSTRATE INTO AND OUT OF A COATING CHAMBER MAINTAINED AT A PRESSUREBELOW ABOUT 20 MICRONS HG ADS., EXPOSING THE FRONT SURFACE OF THESUBSTRATE TO ALUMINIUM VAPORS WHILE IT IS IN THE COATING CHAMBER, THECONCENTRATION OF ALUMINIUM VAPORS BEING SUFFICIENT TO PRODUCE A COATINGHAVING AT LEAST 70 PER-